This invention relates to valve structures and, more particularly, to gate valve structures wherein retraction of a counter plate is delayed relative to retraction of a seal plate to permit pressure equalization and to reduce wear on seal surfaces.
Conventional gate valve structures include a valve housing having a fluid conduit and a valve seat, a seal plate that is movable between an open position and a closed position in the fluid conduit, and an actuator mechanism for moving the seal plate between the open and closed positions. The seal plate engages the valve seat and seals the fluid conduit in the closed position. The seal plate may be moved from the closed position to a retracted position and then moved linearly to the open position.
Gate valves are used in a wide variety of applications. Different applications may involve liquids, gases, and vacuum. Many applications require a long operating life, with frequent cycling between the open and closed positions, and low particulate generation. An example of such an application is in equipment for processing of semiconductor wafers. As semiconductor device geometries decrease in size and circuit densities increase, semiconductor wafers are increasingly sensitive to particulate contamination. Components within the vacuum envelope of the processing chamber, such as gate valves, are potential sources of particulate contamination. Furthermore, the failure of a gate valve may require all or part of a semiconductor fabrication line to shut down, thereby adversely affecting throughput. Accordingly, long operating life and low particulate generation are important gate valve characteristics.
Another potential problem with gate valves relates to the shock and vibration that may occur when the valve is opened or closed. The shock and vibration may damage or disrupt the operation of sensitive equipment in proximity to the gate valve. It has been observed that the shock produced by the operation of prior art gate valves may jar loose the charcoal from the inside of cryogenic vacuum pumps. Accordingly, it is desirable to limit shock and vibration produced by gate valves.
A gate valve having an linearly movable seal plate is disclosed in U.S. Pat. No. 4,052,036 issued Oct. 4, 1977 to Schertler. The seal plate and a counter plate are biased toward each other by leaf springs. The actuator carries a series of rollers which engage recesses in the seal plate and the counter plate. When the seal plate and the counter plate reach a stop position, the actuator continues to move, forcing the rollers out of the recesses and moving the seal plate and the counter plate toward the closed position. The seal plate engages a valve seat, and the counter plate engages a support surface. The counter plate provides support for the seal plate in the closed position and prevents the seal plate from being forced away from the valve seat by a pressure differential across the valve.
When gate valves of this design are opened, the seal plate and the counter plate are retracted simultaneously. This leaves the seal plate unsupported when the seal between the seal plate and the valve seat is broken. In the presence of a pressure differential across the valve, the unsupported seal plate is forced into the rollers and the counter plate. This causes a shock which may damage sensitive components and which may loosen particles in the system. In addition, the elastomer ring on the seal plate may be subjected to scuffing or rubbing as the seal plate is retracted from the valve seat. Such scuffing or rubbing may produce in wear of the elastomer ring and reduce the operating life of the gate valve.
Accordingly, there is a need for improved gate valve structures.
According to a first aspect of the invention, a valve is provided. The valve comprises a valve housing having a fluid conduit and defining a valve seat and a support surface, a seal plate, a counter plate, an actuator for moving the seal plate and the counter plate between an open position and a closed position, and a coupling mechanism operatively coupled between the seal plate, the counter plate, and the actuator. The seal plate is in sealed engagement with the valve seat, and the counter plate is in engagement with the support surface in the closed position. The coupling mechanism retracts the counter plate from the support surface subsequent to retraction of the seal plate from the valve seat as the valve is opened.
In one embodiment, the coupling mechanism comprises one or more seal plate grooves in the seal plate, one or more counter plate grooves in the counter plate, roller elements movably mounted in the actuator, and one or more springs for biasing the seal plate and the counter plate toward retracted positions. The roller elements move along the seal plate grooves and the counter plate grooves as the actuator moves toward or away from the closed position. The seal plate grooves and the counter plate grooves each have a shallow portion and a deep portion. Transitions between shallow portions and deep portions in the seal plate grooves are offset in a direction of actuator movement relative to transitions between shallow portions and deep portions in the counter plate grooves. In one embodiment, the seal plate grooves and the counter plate grooves each have a relatively abrupt transition between the shallow portion and the deep portion. In another embodiment, the seal plate grooves each have a relatively gradual transition between the shallow portion and the deep portion, and the counter plate grooves each have a relatively abrupt transition between the shallow portion and the deep portion.
According to another aspect of the invention, a method is provided for operating a gate valve wherein a seal plate engages a valve seat and a counter plate engages a support surface in a closed position. The method comprises the steps of retracting the seal plate from the valve seat and retracting the counter plate from the support surface subsequent to retraction of the seal plate from the valve seat.
By retracting the seal plate from the valve seat before the counter plate is retracted from the support surface, any pressure differential across the valve is allowed to equalize. Thus slamming of the counter plate against the support surface is avoided when the seal is broken. In addition, the seal plate is retracted from the valve seat substantially perpendicular to the valve seat, and rubbing and wear of the seal surfaces are limited.